The present invention relates generally to the field of high speed data communications, and more specifically, to the field of transmitting high speed data utilizing a low speed data wireless telecommunications medium.
One of the primary standard specifications relevant to the present invention is TIA/EIA/IS-95-A "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System." This code division multiple access (CDMA) industry standard is considered understood by those reasonably skilled in the art of the present invention.
According to the TIA/EIA/IS-95-A standard, a single data channel in the CDMA system can transmit a maximum of 9600 bits (binary digits) of information per second. While this data rate is sufficient for communicating voice information, the data throughput required for digital data transmission is considerably higher. In transmitting digital data, it is often the case that the higher the data rate the better.
One proposed solution combines multiple low data rate channels into a single high data rate channel, thereby exploiting the ability of CDMA systems to combine data signals into a common signal and then later extracting the original data from the common signal. This proposed solution simply modulates the data from each individual data channel by a slightly different phase angle and combines the data from each individual data channel into a high data rate channel. However, by requiring that each data channel be modulated by a modulating signal with a slightly different phase angle, this proposed solution places severe performance requirements upon the analog portion of the CDMA system. For example, if the CDMA system is attempting to combine eight data channels, the difference in the phase angles of the modulating signals cannot be more than 180/8 or 22.5 degrees. In general, the difference in the phase angles of the modulation signals is equal to 180/n degrees where n is the number of individual data channels being combined. In actual practice, due to the difficulties of designing the analog hardware that can perform adequately with such a small phase angle difference, the number of different phase angles used is small. In order to provide a high data rate channel, i.e., combine a large number of low data rate channels, the proposed solution shares the same phase angle between different data channels, leading to a loss of processing gain, hence an overall reduction in the signal to noise ratio.
There is, therefore, a need in the industry for a system addressing these and other related and unrelated problems.